Apparatus for enhancing the detail at the boundary of light and dark areas of a xerographically produced image wherein, after exposure but before development, the photoreceptor is partially recharged using a scorotron having a biased screen between the scorotron corona wires and the photoreceptor which charges photoreceptor areas that are discharged to below some predetermined level up to that level, but to not charge those areas that are already above the predetermined level.
In the xerographic process, the surface of a photoreceptor is charged up to a relatively high level, 1600 volts, for example, at a charging station. Next, the photoreceptor is partially discharged by illuminating it with a light image. The areas that receive light become conductive and conduct the surface charges to ground, while the areas not receiving light remain unaffected and the surface charge remains. Next, at the development station, the surface is brought into contact with toner which is charged to be attracted to the charged (or discharged) areas. Finally, at the transfer station, the toner is transferred to a sheet of paper to make a hard copy.
A characteristic of this process is a boundary effect where the colors on both sides of a sharp boundary are enhanced. For example, at a sharp boundary between dark gray and light gray areas, the dark gray area will be outlined in black at the boundary, and the light gray area will be outlined in white at the boundary.
In some cases, this boundary effect will decrease the usefulness of the resultant image. For example, in x-ray mammography, at the minimum x-ray level that will generate good detail, the tissue appears as a darker gray area against a lighter background, with a sharp boundary at the skin line. Details, such as small tumors and calsifications, that will show up in the bulk of the image will be obscured at the skin line by this darkening effect. This is referred to as "skin line deletion ", and can be described by using a numerical example. Assume that, in a fairly uniform area, a visible line can be seen when there is a difference of 2 volts. That is, a boundary will be visible when there is a change from 50 to 52 volts, the 52 volt side will appear darker than the 50 volt side. However, if the 50 to 52 volt boundary is placed just within the skin line, and the background has a potential of 10 volts, then both the 50 and 52 volt areas will appear black, and the detail will be lost. In general, skin line deletion is caused by potential differences greater than about 25 volts between the edge of the breast and the background outside the breast area. To reduce the skin line boundary, the current approach is to increase the radiation by about 50%, which discharges the entire image to lower levels, thereby reducing the difference between the average tissue and background discharge levels, and allowing more detail to be visible.
To lighten up the image so that details are visible at the skin line, more image radiation is used, but since x-rays themselves are a cancer causing agent, this greater radiation dose is undesirable. What is required, and provided by this invention, is a method for enhancing the image quality at the boundary without increasing the radiation.